SCP-XXXX is to be stored in a 3 m X 3 m cement cell within a steel footlocker which has an electronic lock.

I believe you are missing a dimension here; also, a 3m3 cement cell within a footlocker is weird. Either I’m missing something or this is one huge footlocker. Let me put this into perspective. 3m is equal to about 10 feet, three cubic meters is equal to about 100 cubic feet. So that footlocker is pretty big compared to normal footlockers. You are also missing a comma between “footlocker” and “which”.

it's materials have not been identified.

“Its” not “It’s” when showing possession.

SCP-XXXX-B superficially resemble silver coins

I don’t think you really need the “superficially silver coins” part, or at least put it after the diameter and weight part.

SCP-XXXX was found in the possession of █████ ██████ (designated subject XXXX-1) when the Foundation was alerted to a █████ █████ Hospital [REDACTED] of the CDC.

The last part is very awkwardly worded and not really needed.

Addendum XXXX-1

What?

Addendum XXXX-3

I think that is a given with all SCPs.

Addendum XXXX-5 through 8

These could probably be combined.

All said, I really don't think this is all that good. Things that bring people back from the dead have been done before. Granted, I am rather new here and others probably know what will be successful and what won't, but I wasn't that impressed. Now, of course there of course, is no telling what will happen, so I don't rightfully know.

I've rewritten a few parts that you mentioned weren't clear enough so that they are now simplified. I've also clarified the whole footlocker thing, as it seems to have confused you as to what I was trying to say.

I understand your criticism of the 'things bringing people back from the dead'. That was actually something I redrafted this SCP in mind. Previously, it was a standard resurrect the dead type thing. Now, my intention is for the resurrection to be a by-product of what it does, not the goal. If that wasn't clear, I'd appreciate any thoughts you might have on the matter.

The addition of long and drawn-out addenda has not really countered the fact that this is just a magic item that brings people back to life. In fact I'm not even sure what you're trying to get at in the addenda; it's way too cluttered up with technobabble.

Also, Addendum XXXX-3 is what we call LolFoundation. It's horribly tone-breaking and kills suspension of disbelief.